1. Field of the Invention
The present invention relates to an optical element with a variable focal length and in particular to a focus detecting device equipped with such optical element.
2. Related Background Art
There are already proposed various variable-focus lenses, for example a lens composed of an elastic container of which shape is variable by the pressure of liquid filled therein, such as disclosed in the Japanese Patent Laid-open No. 36857/1980, or a lens utilizing a piezoelectric element such as disclosed in the Japanese Patent Laid-open No. 110403/1981 or 85415/1983. However, the former so-called liquid lens cannot be made compact as it requires a liquid reservoir and a pressurizing device, and is associated with deformation of surface by gravity or vibration. On the other hand, the later lens has a limited variable range of the focal length.
In order to overcome these drawbacks, the present inventors already proposed a variable-focus lens as shown in FIG. 1, comprising a transparent elastic member 21, a glass plate 22, a lateral wall 22, and an aperture plate 24 having an aperture 24a. 21a indicates the surface of the elastic member 21 in the aperture 24a of the aperture plate 24.
FIG. 1A shows an initial state before deformation, wherein the surface 21a of the elastic member in the aperture 24a has a determined molded form such as a plane or a spherical surface. When the aperture plate 24 is moved in a direction A, as shown in FIG. 1B, to press the elastic member 21, whereby the elastic member 21 protrudes from the aperture 24a and the surface 21a of the elastic member 21 in the aperture 24a assumes a more strongly curved state than in FIG. 1A. Consequently said surface 21a can be utilized as a variable-focus lens surface. This structure is advantageous in providing a large variable range of focal length with a small dimension and with a small external force.
In order to adapt the element shown in FIG. 1 into an actual optical system, it is necessary to select the diameter b of the aperture, shown in FIG. 1, as close as the diameter a of the element, thereby minimizing the optically uneffective portion other than the lens surface and thus compactizing the entire optical system.
However, though the surface 21a of the elastic member deforms in a spherical form when the diameter b of the aperture is sufficiently smaller than the diameter a of the element, there appears a drawback of significantly aspherical deformation in the surface 21a with a stronger curvature in the peripheral area thereof in case the diameter b of the aperture becomes closer to the diameter a of the element. In this case the deformation of the elastic member 21 depends solely on Poisson's ratio but not on the elastic modulus. Consequently the form of the elastic member 21 is deformed state remains also the same regardless of the material thereof. It has not, therefore, been possible to obtain, with the conventional structure, a desired surface form with a small structure regardless of the material employed.